Pump drained solar water heating system

ABSTRACT

A pump drained solar water heating system, includes a solar collector; a reversible pump; a reservoir for storing heat transfer fluid; an insulated water storage tank for storing the potable water; piping for fluidically connecting the pump to the solar collector, the heat transfer fluid reservoir; and a heat exchanger; and a controller for operating the pump to transmit fluid in a forward flow or reverse flow direction.

BACKGROUND OF THE DISCLOSURE

Solar water heating systems are made up of components that collect solar energy, transfer the energy to the potable water via a heat exchanger, store the thermal energy, control system operations, deliver hot water where it is needed, and protect the system against freezing. Components can be combined in a variety of ways, but these six basic functions must be met, although some systems are simple enough that passive physics provide the control and motive forces to drive the fluids and heat transfer in the system.

Solar water heating systems can generally be described using the following four terms: direct or indirect and passive or active. Direct systems heat the potable water directly through solar exposure. These systems are typically used in regions with little freeze risk since the potable water is used as the heat transfer fluid and therefore exposed to temperature outside of conditioned space. Indirect systems use a secondary heat transfer fluid to collect and transfer the solar energy to the potable water. The heat transfer fluids used are typically freeze resistant and these systems are better suited for cooler climates. Passive systems use the differential density created by the thermal gradient of the water or heat transfer fluid to move fluids in the system and accumulate heated potable water for usage. Active systems employ pumps to circulate the water or heat transfer fluid for the purpose of heat exchange. Several different system configurations are currently used based on these concepts.

One simple configuration of solar water heater is a solar batch heater. The system consists of a south facing, roof-mounted tank enclosed in an insulated box covered with glass. Cold water is supplied to the bottom of the tank. When the water is heated by the sun, it rises to the top of the tank. The heated water is removed from a port located at the tank top. The mains supply water pressure is used to push the hot water from the tank as the cold is supplied to the bottom. The system is a direct passive system, meaning the potable water is heated directly and no water circulation is directed beyond that which is created by the thermal density driven gradient. Because of its relatively low cost and simplicity, the solar batch heater configuration works well for those living in moderate climates with good sunshine available.

Another relatively simple, passive solar water heater system is the thermosyphon system in which the water tank is located directly above the collector. Thermosyphon systems work on the principal of heat rising. In a direct system, the potable water enters the bottom of the collector and rises to the tank as it warms. The insulated tank at the top of the system stores the water prior to usage. In colder climates, an antifreeze solution, such as propylene glycol, is used in an indirect closed solar loop, which uses a heat exchanger to pass energy to the potable water. Since water is supplied directly to the roof mounted storage tank, design provisions must be made to avoid freezing of the exposed mains supply in the attic and roof.

A third solar water heating system is an active, direct system. This system is typically used in tropical settings where freezing never occurs. A solar storage tank is combined with a solar thermal collector. A pump is used to circulate the potable water through the collector and to the storage tank which serves as a preheat tank for a separate, existing water heater which includes an auxiliary heater for supplemental heating.

Another solar water heating system is an indirect, active system, which uses a pressurized heat transfer fluid to heat the potable water. In this system, incoming potable water is routed to the storage tank. Heat transfer fluid circulates from the collectors through a heat exchanger at the storage tank, and then is actively pumped back through the collectors. The potable water is heated via the heat exchanger at the storage tank. Supplemental heaters are provided in the storage tank or the storage tank serves as a preheat for an existing water heater.

Still another solar water heating system is an indirect, active drainback system. The heat transfer fluid is typically distilled water. When the system is not pumping, the solar collector is empty and the distilled water is stored in an appropriately sized reservoir tank located just above the storage tank. When the pump turns on, the distilled water is circulated from the reservoir back through the collector and heat exchanger, passing heat to the potable water in the storage tank. When the pump shuts off again, the distilled water drains back into the reservoir. As a result, a collector must always be higher than the storage tank, and there must be sufficient continuous slope in the piping to ensure complete drainage and prevent freezing of the heat transfer fluid.

A problem with the drainback system is it requires the fluid line to be configured to allow complete drainage due to gravity only. This configuration requires the installer to mount all plumbing so as to avoid trapping of any fluid which would potentially freeze and rupture the plumbing. Another problem with this drainback system is that larger pumps usually have to be used, especially if water is being pumped up two stories or more, since the drainback pump has to lift the distilled water to the height of the solar collector.

Thus, it is desirable to develop a new and improved indirect active system for a solar water heater, which overcomes the above-mentioned deficiencies and others while providing better and more advantageous overall results.

SUMMARY OF THE DISCLOSURE

A solar water heating system comprises a solar collector panel in fluid communication with a heat exchanger which is in thermal communication with the water storage tank. A reversible pump is used to circulate the heat transfer fluid between the heat exchanger and the solar collector panel. Under normal operation, the heat transfer fluid is circulated in a first direction filling the system and transferring heat energy from the collector to the water storage tank via the heat exchanger. When conditions warrant, such as for example to avoid freezing or overheating of the fluid, the pump reverses the flow to remove the heat transfer fluid from the collector to a storage reservoir.

Specifically, a positive displacement reversible pump allows for the normal flow operation and the ability to remove the fluid from the solar collector. The diameter of the pipes linking the collector panel, the heat exchanger and the pump is of the appropriate dimension to maintain a capillary action which allows the fluid to be pumped from the pipe under suction. The pipe diameter must also be sufficiently small to avoid entrainment of air into the fluid, which would prevent complete evacuation of the fluid.

One aspect of the disclosure is a solar water heating system, including: an insulated water storage tank for storing the water to be heated; a solar collector; a reversible pump; a reservoir for storing heat transfer fluid; a heat exchanger in thermal communication with the storage tank for transferring the thermal energy from the heat transfer fluid to the water in the tank; piping for connecting the pump, the collector, the heat transfer fluid reservoir, and the heat exchanger in fluid communication; and a controller with a back up or reserve power supply for briefly operating the controller and pump during a loss of power; wherein the controller senses certain system temperatures and selectively operates the pump in a forward flow or reverse flow direction as a function of the sensed temperatures.

Another aspect of the disclosure is that the system automatically removes the heat transfer fluid from the solar collector, upon detection of conditions that could otherwise expose the heat transfer fluid to temperature extremes which could degrade the fluid resulting in increased corrosion to the system components and possible leakage failure or otherwise damage the collector or other elements of the system.

Another aspect of the disclosure is that the system pumps the heat transfer fluid to evacuate the plumbing system, and avoids the disadvantages of a pipe installation which relies on gravity to drain the fluid from the collector and piping system.

Another aspect of the disclosure is that deionized water can be used as the heat transfer fluid. Deionized water has improved heat transfer properties, is less expensive and less toxic than traditional heat transfer fluids, such as propylene glycol. Using deionized water may also allow usage of a single wall heat exchanger between the heat transfer fluid and the potable water, depending on local plumbing code requirements.

Yet another aspect of the disclosures is the installation time for the system would be reduced because the installer would simply fill the reservoir with fluid and prime the system. In contrast, traditional closed systems require a secondary pump and a valve for pressurization of the system.

Still other aspects of the disclosure will become apparent upon a reading and understanding of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustrating a pump drained solar water heating systems in accordance with a preferred embodiment of the present disclosure;

FIG. 2 is a schematic illustrating the pump drained solar water heating system of FIG. 1 in a pump back configuration;

FIG. 3A is a side elevational highly schematic view of a pressure vacuum valve of the solar water heating system of the present disclosure illustrating the valve in its neutral state;

FIG. 3B is a side elevational highly schematic view of a pressure vacuum valve of the solar water heating system of the present disclosure illustrating the valve in its forward flow state;

FIG. 3C is a side elevational highly schematic view of a pressure vacuum valve of the solar water heating system of the present disclosure illustrating the valve in its reverse flow state;

FIGS. 4A and 4B are simplified schematic circuit diagrams of power control circuits for the pump motor of the system of FIG. 1; and

FIG. 5 is a control logic diagram for the system of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a solar water heating system 10 in accordance with a preferred embodiment of the present disclosure is shown. The system 10 includes a solar collector 12 which may be of conventional design which collects and provides heat to a heat transfer fluid that then transfers the heat from the collector 12 to water contained in a conventional insulated water heater storage tank 14 having a cold water inlet 13 and a hot water outlet 15. A thermostatic mixing valve 17 is provided at the hot water outlet 15 which mixes cold water from the cold water supply line 13 with the hot water from the hot water outlet line 15 to provide the user with water at the user selected water temperature. Such mixing valves are typically used in solar hot water heating systems because it is desirable to store water in the tank at temperatures which may be higher than that desired for use by the user, for increased energy storage. This is because solar energy is only available periodically over the course of a day, e.g., during daylight hours, and the intensity of the sunlight varies even during such hours, so in order to take the fullest advantage of the energy when it is available, the water in the heater storage tank may be allowed to reach a higher temperature than that desired by the user when turning on the hot water. For example, the maximum temperature for the water in the tank may be pre-set at a value on the order of 160 degrees F., while the user may prefer a setting in the 110-120 degrees F. range. The mixing valve blends cold water as needed with the hot water to properly limit the temperature of the water from the water heater system to the temperature selected by the user, effectively increasing stored hot water capacity. Mixing valve 17 may be any one of many commercially available thermostatic mixing valves.

The heat transfer fluid used in solar water heating systems is typically either water or a combination of propylene glycol and water. Water is typically the best heat transfer fluid for both holding and transferring heat. Deionized water can be used as the heat transfer fluid. Deionized water has improved heat transfer properties, is less expensive and less toxic than traditional heat transfer fluids, such as propylene glycol. Using deionized water may also allow usage of a single wall heat exchanger between the heat transfer fluid and the potable water. However, water does freeze. Propylene glycol, mixed with water, is the industry standard heat transfer fluid where freezing is a hazard. Since propylene glycol is nearly non-toxic, heat exchangers designed for this fluid need only be single wall construction, depending on local code requirements.

Conventional drain back solar water heater systems rely on gravity to remove the heat transfer fluid from the solar collector when freezing or overheating conditions arise. Referring again to FIG. 1. in accordance with the present disclosure, the disadvantages of relying on gravity for removing heat transfer fluid from the solar collector are overcome by use of a positive displacement reversible pump 16 is operative to selectively circulate heat transfer fluid through the solar water system in a first direction for operation in the heat exchange mode and in a reverse direction to remove or drain the fluid from the solar collector to a fluid storage reservoir.

In FIG. 1, operation in the heat exchange mode providing heat to the storage water tank is shown. The flow of fluid is circulated filling the system and transferring heat energy from the solar collector to the water storage tank via the heat exchanger. Specifically, the controller operates the pump 16 in a first or forward direction to pump the heat transfer fluid in the direction of arrows 34, 36 via piping 18 through the collector 12 through the heat exchanger fluid reservoir 32 to the storage tank heat exchanger 19. A baffle 38 is positioned in the insulated heat exchanger fluid reservoir 32 to control the fluid drainage into the reservoir and reduce fluid aeration. As the heat transfer fluid passes through the storage tank heat exchanger, heat is transferred to the potable water in the storage tank 14. The tank 32 is located in a conditioned space and is vertically close to the collector to reduce head pressure.

Referring now to FIG. 2, when conditions warrant, the controller activates the pump in a reverse direction, and the pump reverses fluid flow direction, removing or draining the heat transfer fluid from the collector 12 to the heat transfer fluid storage reservoir 32, thus avoiding overheating of the fluid. The heat transfer fluid travels in the direction of arrows 40, 42 through the piping 18 from the storage tank to the heat transfer fluid reservoir. The controller operates the pump 16 in a reverse flow direction for a period sufficient to evacuate the heat transfer fluid from the collector to the heat transfer fluid reservoir. Such a pump back operation is initiated, for example, when the fluid temperature at the collector has reached a maximum or minimum predetermined temperature or power to the system has failed.

A pressure vacuum valve 30 is used where the fluid circuit opens into the heat transfer fluid reservoir. The purpose of the valve is to prevent unintended draining of the heat transfer fluid from the fluid circuit. When the pump is pumping heat transfer fluid from the reservoir to the collector, sufficient pressure is produced to open the pressure valve and allow flow into the reservoir. When the pump stops the valve closes preventing drainage from the fluid line. Under conditions which warrant evacuation of the heat transfer fluid from the lines and collector, such as freezing, excessive temperature or a power failure, the pump reverses and creates a pressure differential sufficient to open the vacuum valve and allow air to enter the system as the fluid is pumped from the collector and line to the reservoir. While the system will function without the pressure vacuum valve, use of such a valve to keep fluid in the lines when the pump is idle under conditions which do not warrant fluid evacuation reduces the thermal shock experienced when introducing fluid, which has cooled, into a heated collector during periodic cycling of the pump.

FIGS. 3A, 3B and 3C schematically represent an illustrative embodiment of pressure vacuum valve 30. Valve 30 includes a housing 31 which defines a central channel 52 open at opposing ends. An inner liner 33 of having an inner diameter less than the inner diameter of housing 31 defines a shoulder 35 which provides a valve seat for valve member 44. The valve member 44 has an I-shaped cross-section, comprising a lesser plate 44 a and a greater plate 44 b joined by web member 44 c. A plate 48 with a central opening 49 for loosely receiving web member 44 c is positioned between plates 44 a and 44 b for movement therebetween plates guided along 44 c. Plate 48 further includes a plurality of openings 50 to permit fluid flow through plate 48. Compression coil spring 46 which circumscribes member 44 c, is sandwiched between plate 44 a and plate 48 to bias plate 48 against plate 44 b. Holes 50 are positioned such that when plate 48 abuts plate 44 b, holes 50 are blocked by plate 44 b preventing flow there through and when plate 44 b is spaced from plate 48 holes 50 are open to permit flow there through. A second compression coil spring 54 which circumscribes spring 46, is sandwiched between end wall 30 b and plate 48 to bias plate 48 into seating engagement with shoulder 35 to prevent fluid flow between plate 48 and shoulder 35 when so seated.

FIG. 3A, shows the pressure vacuum valve 30 in its neutral position or state, which it assumes when the pump is off. In this position, spring 46 causes plate 44 b to abut plate 48 closing holes 50 and spring 54 causes plate 48 to be seated against shoulder 35. In this state valve 30 blocks flow in both directions.

FIG. 3B shows valve 30 in its forward flow state, which it assumes when pump 16 is operating in the forward flow direction to circulate the heat transfer fluid through the system from the collector to the heat exchanger to transfer heat from the collector to the water being heated in the tank. In this forward flow state, a fluid pressure in the direction shown by the arrows through opening 56 is sufficient to overcome the bias of spring 46, and move plate 44 b away from plate 48 permitting flow through the valve in the forward direction (downward in FIG. 3A) through holes 50 which are now unblocked into a central channel 52 of the valve.

FIG. 3C shows valve 30 in its reverse flow position or state which it assumes when pump 16 operates in the reverse flow direction described with reference to FIG. 2, to move the heat exchange fluid from the collector to the heat exchange fluid storage reservoir. With pump 16 operating in this direction, fluid flow through valve 30 is shown by the arrows in FIG. 3C to be in the upward direction through the valve. Pressure from the pumped fluid is sufficient to overcome the bias of spring 54 causing plates 44 b and 48 to move vertically relative to shoulder 35 thereby unseating plate 48 and allowing fluid to flow through the valve in the reverse direction through the gap between plate 48 and shoulder 35 and up through opening 56.

The piping 18 is the conduit through which heat transfer fluid travels between the collector 12 and the water storage tank heat exchanger 19. The piping extends around the exterior of the tank in a coil wrap heat exchanger configuration 19. The piping must be properly sloped to allow for proper flow, dropping at least ¼ inch per linear foot. A diameter of the system line pipe also defines the ability of the pipe to maintain a capillary action which allows the fluid to be pumped from the pipe under suction. The pipe diameter must be sufficiently small to avoid entrainment of air into the fluid, which would prevent complete evacuation of the fluid. A pipe diameter on the order of ⅜ inches provided satisfactory results in the embodiment of FIG. 1. Due to exposure to freeze-thaw cycles and potentially very hot fluids, copper is the preferred pipe material. As is well known in the art, various grades of copper are used for different portions of the solar system. However, other suitable materials can be used without departing from the scope of the disclosure. Pipe joints are typically soldered but could include mechanical crimping, barbed springs or clamping for retention and flat gaskets or diametral seals for fluidic integrity of the connections.

The storage tank heat exchanger could also be implemented in a variety of alternative configurations to provide an efficient heat exchange relationship between the fluid and the water in the storage tanks.

Pipe insulation is also used for energy performance and freeze protection. Suitable insulation includes elastomeric insulation which has an R-value of 3.5 per inch, is used for heating and cooling systems, and is preferred for exterior and interior applications. Fiberglass insulation, with an R-value of 3 per inch, is suitable for interior applications. Preferably, such insulation is applied to all exposed lengths of pipe.

As hereinbefore briefly described, pump 16 is selectively operated in a first direction to circulate the heat transfer fluid through the heat exchanger to heat the water in the tank, and also under certain circumstances to operate in a reverse direction to drain the fluid from the collector. Referring again to FIG. 1, a controller 20 determines when sufficient solar energy is available and controls system operation accordingly. The controller is operative via an electrical wire connection 27 to control the pump 16 that pushes fluid through the system. The controller acts as a differential temperature controller and works with two temperature sensors 22, 24 and is connected to sensor 22 via wire 23 and is connected to sensor 24 via wire 25. Sensor 22 is connected to the collector and is positioned near the hottest point in the collector. In the illustrative embodiment, sensor 22 is a thermistor sensor which is band clamped to the copper fluid line at the top of the collector to sense the temperature of the fluid in the line at that point which is representative of the highest temperature of the fluid in the collector. Sensor 24 is an thermistor sensor that mounted to the storage tank to project into the interior of the tank and is positioned near the bottom of the storage tank 14 so as to be in thermal contact with the water stored in the tank to sense the temperature of the water in the tank. The power to energize the controller and pump under normal operating conditions is provided by an external power source such as a 120 or 240 volt ac power supply from a utility company, or an integrated power source such as a photovoltaic cell collector 21. When an external power source is utilized for the pump and controller, a reserve power supply, such as a battery 29, is preferably included to provide power operate the pump in the reverse direction to evacuate the heat transfer fluid from the system in the event of a failure of the primary power supply.

When the controller determines that the temperature of the water in the tank as sensed by sensor 24 is less than the Maximum Water Storage Temperature Reference value and the temperature differential between the sensed temperature of the fluid in the collector sensed by sensor 22 and the sensed temperature of the water in the tank sensed by sensor 24 exceeds a pre-set “Pump On” reference value, the controller turns on the pump for operation in the first or heat exchange direction. Unless overridden due to other circumstances to be hereinafter described, once turned on, the pump remains on until the temperature of the water in the tank as sensed by sensor 24 is no longer less than the maximum storage tank water reference in order to prevent overheating of the water in the storage tank, or until the sensed temperature differential between the fluid in the collector and the water in the tank drops to a reference “Pump Off” value, at which time the controller turns off the pump. These reference differential temperature values are pre-set by the installer in accordance with the specific characteristics of the system to enable the system to heat efficiently without cycling too frequently. A reference Pump On value on the order of 18 degrees F. and a reference Pump Off value on the order of 9 degrees F. should work satisfactorily for typical installations but other values could be similarly employed. A Maximum Water Storage Temperature Reference value on the order of 160 degrees F. is suitable for typical installations but other values could be similarly employed.

In addition to controlling the pump in response to the temperature differential between the temperature of the fluid in the collector and the temperature of the water in the tank for efficient heating, the controller also operates to reverse the direction of the pump to drain the fluid from the collector to the reservoir under certain conditions to avoid damage to the system. For example, the controller is arranged to operate the pump in the reverse direction (that is, in the direction opposite to the direction for circulating the fluid through the heat exchanger) to drain the collector in response to the detection by sensor 22 of a temperature of the fluid in the collector greater than a pre-set Maximum Collector Reference Temperature to prevent overheating of the fluid in the collector which could damage the collector or and/or cause deterioration of the fluid. A value for the Maximum Collector Reference Temperature that is high enough for efficient heating of the water in the storage tank, but low enough to avoid exceeding the boiling point of the fluid is desirable. Similarly, the controller operates the pump in the reverse direction in response to the detection by sensor 22 of a fluid temperature less than a pre-set Minimum Collector Reference Temperature to prevent damage due to freezing of the fluid in the collector. A Maximum Collector Reference Temperature value on the order of 210 degrees F. and a Minimum Collector Reference Temperature value on the order of 35 degrees F. are suitable for typical installations, but other values could be similarly employed to avoid overheating or freezing of the fluid in the collector. It is also desirable to drain the fluid from the collector in the event of a power failure. Upon detection of such a failure, the system is arranged to activate a reserve power source operative to run the pump in the reverse direction to drain the fluid from the collector to the reservoir. In the illustrative embodiment operation of the pump in the reverse direction is timer controlled, that is, once operation in the reverse direction is initiated, the pump continues to run in that direction for a predetermined time sufficient to assure that the fluid has been sufficiently drained from the collector. Alternatively, a float switch or other level sensing arrangement could be used to turn of the pump off when the fluid level in the reservoir reaches a level signifying that the collector has been adequately drained.

In the illustrative embodiment, the pump 16 is driven by a reversible dc motor. FIGS. 4A and 4B, are a simplified schematic circuit diagrams of embodiments of pump motor power control circuits which provide for switching to the backup battery 29 in the event of a power failure. Referring first to FIG. 4A, the power for the circuit under normal operating conditions is provided by a standard 120 or 240 volt AC power source 102 connected to a DC power supply circuit 104 which converts the AC signal to a DC signal to drive the DC pump motor 106 using conventional AC to DC converter circuitry which is also arranged to reverse the polarity of the DC output signal as required in response to the system controller to drive the reversible pump motor in the desired direction. A backup power relay 108 is employed to switch the back up battery 29 into the circuit to energize motor 106 in the event of a main supply power failure. The coil for the relay is selectively energized by the system controller 20. The normally closed contacts connect motor 106 across the primary power supply 104 and the normally open contacts connect the motor 106 across the backup battery 29. Controller 20 monitors the state of main supply 102 by a voltage sensor 110. When the sensed voltage drops below a predetermined level indicative of a power failure, controller 20 energizes the coil of relay 108, and sustains energization until the main power is restored. The system controller 20 is provided with a back up battery supply as well to sustain operation of the controller in the event of a power failure in a conventional manner. By this arrangement, when power is available from the main power supply, the relay coil will be de-energized and the pump motor will be driven by the DC power supply 104. However, if no power is available from the main source 102, the relay coil will be energized by the backup battery 29 and the relay 108 will switch to its normally open position and the motor 106 will be driven by the back up battery 29. The connection of the battery to the pump motor 106 via the relay will be arranged such that the polarity of the signal applied to the motor will drive it in the pump out direction, so that in the event of a power failure, the collector will be drained. Fill control switch 112 opens to de-energize motor 106 when the collector has been sufficiently drained. As hereinbefore described, in the illustrative embodiment operation of the pump in the reverse direction is timer controlled, that is, once operation in the reverse direction is initiated, the pump continues to run in that direction for a predetermined time sufficient to assure that the fluid has been sufficiently drained from the collector. Alternatively, switch 110 could be a float switch or other level sensing arrangement which opens when the fluid level in the reservoir reaches a level signifying that the collector has been adequately drained.

An alternative back up power supply circuit is illustrated in FIG. 4B. As in FIG. 4A, power for the circuit under normal operating conditions is provided by standard 120 or 240 volt AC power source 102 connected to DC power supply circuit 104 and a backup power relay 108 is employed to switch the back up battery 29 into the circuit to energize motor 106 in the event of a main supply power failure. However, in this circuit, the coil for the relay 108 is across the main power supply 102. The normally closed relay contacts connect the battery 29 to motor 106 and the normally open contacts connect the DC power supply 104 to motor 106. By this arrangement, when power is available from the main power supply 102, the relay coil will be energized and the pump motor will be driven by the DC power supply 104. However, if no power is available from the main source 102, the relay coil will be de-energized and the relay will switch to its normally closed position and the motor 106 will be driven by the back up battery 29. The connection of the battery to the motor via the relay will be arranged such that the polarity of the signal applied to the motor will drive it in the pump out direction, so that in the event of a power failure, the collector will be drained. In this embodiment, switch 110′ is a float switch which opens when the fluid level in the reservoir 32 (FIG. 1) reaches a level signifying that the collector has been adequately drained. An advantage of the circuit of FIG. 4B is that the collector will be drained even if the controller 20 should become inoperative.

Referring now to FIG. 5, a control logic diagram for the controller is illustrated. At the start position 60 the system is powered on, but the pump is turned off. Inquiry 62 determines if the temperature of the water in the storage tank sensed by sensor 24 (Storage Tank Temp) is greater than the Maximum Water Storage Temperature reference value (max set temp). If “Yes” the pump is turned off (Block 64), which causes the pump to stop, halting the circulation of the fluid, and the system proceeds to Inquiry 66. If “No” at Inquiry 62, the system proceeds directly to Inquiry 66.

Inquiry 66 determines if the temperature of the fluid in the collector sensed by sensor 22 (“Collector temp”) either exceeds the pre-set Maximum Collector Reference Temperature value (“max temp”) or is less than the pre-set Minimum Collector Reference Temperature value (“min set temp”). If “Yes” the pump is run in the Reverse Direction (Block 68) to evacuate the collector by draining the fluid from the collector to the reservoir and the system returns to Inquiry 62.

In response to a “No” at Inquiry 66 the system moves to Inquiry 70 which determines if a loss of power has been detected. If “Yes” the reserve power supply is activated (Block 72) and the pump is operated in the reverse direction to drain the fluid from the collector to the reservoir (Block 68). For example, a battery 29 used to provide power needed to evacuate the heat transfer fluid from the system during a power outage, is switched into the system to energize the pump.

In response to a “No” to Inquiry 70 the system continues to Inquiry 76 which determines if the temperature differential between the temperature of the fluid in the collector, sensed by sensor 22, and the temperature of the water in the storage tank sensed by sensor 24 (“Collector-tank temp delta”) is greater than the Pump On value (“set pump on value”). If “Yes”, the system operates the pump in the forward direction (heat exchange direction, to circulate the fluid from the collector through the heat exchanger to heat the water in the storage tank (Block 78) and the system returns to Inquiry 62.

If the response to Inquiry 76 is “No”, the system proceeds to Inquiry 80 which determines if the temperature differential between the temperature of the fluid in the collector, sensed by sensor 22, and the temperature of the water in the storage tank sensed by sensor 24 (“Collector-tank temp delta”) is less than the Pump Off value (“set pump off value”). If “Yes”, the controller then stops or deactivates the pump (Block 82, and the system returns to Inquiry 62. If the response to Inquiry 80 is “No” the system returns to Inquiry 62.

The system removes the heat transfer fluid from the collector, thus avoiding exposure to excessive temperature which degrades the fluid resulting in increased corrosion to the system components and possible leakage failure. Pumping the heat transfer fluid to evacuate the plumbing system, avoids the disadvantages of a pipe installation which relies on gravity to drain the fluid from the collector and piping system.

The disclosure has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. For example, in order to be rapidly responsive to the user needs for hot water, a thermistor or a bimetal controlled electric heating element might be provided in the upper region of the water storage tank to supplement the solar heater. It is intended that the disclosure be construed as including all such modifications and alterations. 

1. A solar water heating system, comprising: a solar collector; a reversible pump; a reservoir for storing heat transfer fluid; a heat exchanger for transferring heat from the heat transfer fluid to potable water; an insulated water storage tank storing said potable water; piping for connecting said pump, said collector, said heat transfer fluid reservoir, and said heat exchanger in fluid communication; and a controller for operating the pump to transmit fluid in a forward flow direction for heating the water and in a reverse flow direction to drain the fluid from the collector to the reservoir.
 2. The solar water heating system of claim 1, further comprising a sensor connected to said collector for determining temperature of the fluid in said collector.
 3. The solar water heating system of claim 2, further comprising a second sensor connected to said water storage tank for sensing temperature of the water in said storage tank.
 4. The solar water heating system of claim 2, wherein said controller is connected to said first sensor.
 5. The solar water heating system of claim 3, wherein said controller is connected to said second sensor.
 6. The solar water heating system of claim 5, wherein when said controller detects a temperature differential, in the range of 9° F. to 18° F. between the temperature sensed by said first and second sensors, said controller activates said pump in said forward direction.
 7. The solar water heating system of claim 6, wherein when said controller detects a temperature differential between said first and second sensors which is less than 9° F., said controller deactivates said pump.
 8. The solar water heating system of claim 5, wherein said second sensor is operative to detect a temperature greater than a maximum reference water temperature and said first sensor is operative to detect a temperature greater than a maximum reference collector fluid temperature and said controller is operative in response to said detection by said first sensor or said second sensor to activate a pump flow reversal to evacuate the heat transfer fluid from the system to the reservoir.
 9. The solar water heating system of claim 4, wherein said first sensor detects a minimum set level which activates a pump flow reversal which evacuates the heat transfer fluid from the system to the reservoir.
 10. The solar water heating system of claim 1, further comprising a reserve power supply for energizing the system and wherein said controller is operative in response to a loss of externally supplied power to activate said reserve power supply and to operate the pump in a reverse direction to evacuate the heat transfer fluid from the system to the reservoir.
 11. The solar water heating system of claim 1, wherein said heat transfer fluid storage reservoir comprises a baffle to control fluid drainage into said reservoir.
 12. The solar water heating system of claim 1, further comprising a pressure-vacuum valve adjacent said heat transfer fluid storage reservoir. 